Biodegradable polymers are finding use as environmentally-friendly alternatives for many common plastics in typical applications, such as in packaging materials and bottling.
One problem with many biodegradable polymers, for instance polylactide is the poor melt strength of the polymer. The poor melt strength of polylactide leads to difficulties in subsequent melt processing, such as extrusion, themoforming, blow molding and foaming. Acrylic copolymers have been used as process aids in PVC and polystyrene blends, such as in U.S. patent application Ser. No. 11/053364, however it would not be obvious to use similar processing aids in a polylactide or biodegradable polymers to achieve an improved melt strength. In polycarbonate, for example, no significant improvements in melt strength were noted with the use of an acrylic copolymer processing aid.
The problems with melt strength are compounded for material that has a thermal history (has been previously melt processed—such as a regrind, or recycled material). Polymer chains possess a memory, and the properties often change following heat processing. Recycled or reprocessed materials generally have an even lower melt strength than virgin materials, which can be due to a lower molecular weight, changes in polymer molecule interactions, etc.. The poor melt strength of reprocessed polylactide leads to difficulties in subsequent melt processing, such as extrusion, themoforming, blow molding and foaming.
Several uses of recylced PLA are know in the art. For example, JP 2001253964 and JP 2001252968 describe the blending of recyled PLA with virgin resin for the purposes of producing foamed material, but neither mentions the use of acrylic addivies.
In a similar manner, PLA that has not been dried will lead to hydrolylsis and a reduction in molecular weight during procesing. This results in a decrease in the melt strength of the material. The drying of the material is an expensive, time-consuming process that currently is done to prevent the problems associated with water in the PLA during processing.
U.S. 60/860375 and US 2007-0179218 disclose that the addition of small levels of certain acrylic copolymers to a dried, virgin biodegradable polymer such as polylactide can greatly increase the melt strength of the polymer.
Surprisingly, it has now been found that improvements in melt strength, melt extensibilty, melt elasticity and improved draw-down ratio can be achieved in compromised PLA to an extent even greater than that found with virgin material. The ability of low levels of acrylic additives to not only improve properties of compromised PLA, but to provide properties similar to or even better than vigin material was unexpected.